GitLab: make getFileLink() more robust

No longer assume the URL ends on a slash.
GitLab: reorder version checks in code

This is a cosmetic change: I like ordering the versions from lowest
to highest better than the other way around.
GitLab: update documentation

Put better readable URL examples in documentation.
Add GitLab unit test
GitLab: fix file link for version 4.2

This was suggested in a comment to commit
73249eeac564567f86d8e7d70cf85efa5e5f0b5b by Nick Oliver (PixnBits).

[FIXES JENKINS-25568]
Adjust GitLabTest for older (2.2.x) API